A humanoid high tech alien robot is being chased by an enemy through space and is partially destroyed as it fights in earth. With only his head still in one piece, it manage to merge symbiotically with a human and make an armor with some kind of nano tech. This human in particular is very bad at shooting and as he is a typical bad boy and is used to gang fights with crowbars and stuff. So the helmet IA design a a well-balanced club or something so he can fight off the enemy. At the end, it realizes it is now an unique being.
Sadamitsu the Destroyer (破壊魔定光, Hakaima Sadamitsu)
The story stars Tsubaki Sadamitsu, a young delinquent and leader of the Corpse Gang, who after a chance encounter with an alien robot is forced to take up the task of capturing intergalactic criminals named Ryūkei-dai (流刑台, lit. "Exiled One"). His job is made harder with the appearance of the Vulture, a last-ditch effort to keep the Ryūkei-dai under control.
The head is all that is left of a sentient robot that is trying to capture escaped alien convicts and teams up with the leader of a local motorcycle gang (who wears the head like a helmet) to carry on it's work when it's body is destroyed.
When he wears the helmet, the protagonist gets armour which is gradually upgraded through the series.
He also gets a bokken, which is essential a blunt sword.